1. Field of the Invention
This invention relates to a fluorescence observing apparatus used in a microscope or an endoscope.
2. Description of Related Art
It has been carried out that a specimen, such as living tissue, is irradiated with exciting light and thereby fluorescent light produced from the specimen is used for observation. This fluorescence observation requires the use of an excitation filter unit for transmitting only exciting light with particular wavelengths, of illuminating light, and an absorption filter unit for transmitting only fluorescent light produced from the specimen by irradiating the specimen with the exciting light to block the exciting light.
For example, an endoscope for fluorescence observation is set forth in Japanese Patent Kokai No. Hei 10-239517. This fluorescence observing endoscope apparatus is designed to carry out the fluorescence observation in such a way that the excitation filter unit transmits light with wavelengths less than 460 nm, with which a living body is irradiated to produce fluorescent light, and the absorption filter unit transmits light with wavelengths of 480–600 nm.
In general, the fluorescent light produced from the specimen is faint, and it is important that only the fluorescent light is efficiently taken out. This is governed by the performance requirements of the excitation filter unit and the absorption filter unit.
FIG. 1 shows the relationship that a specimen is irradiated with exciting light 1 and thereby fluorescent light 2 with longer wavelengths than the exciting light 1 is produced for use in the fluorescence observation. In this relationship, it is desirable that an excitation filter unit 3 transmits the greatest possible deal of exciting light and the absorption filter unit completely cuts off the exciting light and transmits the greatest possible deal of fluorescent light. For this purpose, it is necessary that a space C between a half-value wavelength A on the long-wavelength side in the region of wavelengths transmitted by the excitation filter unit 3 (which is hereinafter referred to as “the half-value wavelength on the long-wavelength side of the excitation filter unit 3”) and a half-value wavelength B on the short-wavelength side in the region of fluorescent wavelengths transmitted by the absorption filter unit 4 (which is hereinafter referred to as “the half-value wavelength on the short-wavelength side of the absorption filter unit 4”) is as narrow as possible and the wavelength regions do not overlap.
The reasons why the space between the half-value wavelength on the long-wavelength side of the excitation filter unit and the half-value wavelength on the short-wavelength side of the absorption filter unit is provided are that (1) the stability of spectral characteristics of the filters is not complete and (2) there is a limit to the number of layers of each filter in fabrication.
Reason (1) is based on the fact that since a conventional filter is made by a vacuum evaporation method, film density is not complete, and moisture is absorbed into, or discharged from, a multilayer film because of ambient humidity so that the spectral characteristic curves are shifted. In this case, a shift of approximately ±5 nm is supposed. Even with this shift, the half-value wavelength on the long-wavelength side of the excitation filter unit and the half-value wavelength on the short-wavelength side of the absorption filter unit must not overlap, and thus the space between them needs to be widened in design.
In Reason (2), the number of layers is increased and thereby the spectral transmission characteristic curves of the half-value wavelength on the long-wavelength side of the excitation filter unit and the half-value wavelength on the short-wavelength side of the absorption filter unit can be made to rise steeply so that the transmission areas of the two filters are hard to overlap.
In the vacuum evaporation method which has been used from the past, however, the problems of a fabrication error and intimate film deposition are produced and hence, in fact, film deposition is limited to about 50 layers.